A study on the antagonism of the intestinal inhibitory effects of morphine and clonidine by yohimbine in mice

In mice, clonidine administered subcutaneously caused a dose-dependent inhibition of the intestinal motility as assayed by the movement of a charcoal meal. This inhibitory effect of clonidine was antagonized dose-dependently by prior subcutaneous or intracisternal administration of yohimbine. However, yohimbine given intracerebroventricularly was ineffective in antagonising the intestinal inhibitory action of clonidine. Clonidine administered centrally, either intracisternally or intracerebroventricularly, caused a dose-dependent inhibition of intestinal motility. Clonidine given by the intracisternal route appeared to be more effective than by the intracerebroventricular route. Centrally administered clonidine was antagonized by prior subcutaneous administration of yohimbine. The antagonism was related to the doses of yohimbine given. Subcutaneously administered morphine caused a dose-dependent inhibition of intestinal motility and this effect was antagonized by prior subcutaneous administration of yohimbine. However, administration of yohimbine centrally, either intracisternally or intracerebroventricularly, did not affect the intestinal inhibitory action of morphine. On the other hand, morphine injected centrally, either intracisternally or intracerebroventricularly, dose-dependently inhibited the motility of the intestine; such inhibitory action was antagonized by prior subcutaneous administration of yohimbine. The present study suggests that clonidine inhibits intestinal motility at both central and peripheral sites through alpha 2-adrenoceptors. Morphine also inhibits intestinal motility by both central and peripheral mechanisms but it appears that yohimbine only modifies the peripheral aspect of morphine's action.     

THE POSSIBLE INVOLVEMENT OF ADRENOCEPTORS IN THE INTESTINAL EFFECT OF MORPHINE IN MICE

The inhibitory effect of morphine on intestine was observed by following the intestinal transit of a charcoal meal. This inhibitory effect of morphine was antagonized by naloxone. In addition, the inhibitory effect of morphine was also suppressed by prior administration of yohimbine and phentolamine. However, prazosin, a selective alpha 1-adrenoceptor antagonist, had no effect on the inhibitory effect of morphine on intestinal transit. Furthermore, prior administration of propranolol did not alter this effect of morphine. These adrenoceptor antagonists by themselves, at the doses used, had no effect on the rate of intestinal transit of a charcoal meal in mice. These results suggest that alpha 2-adrenoceptors may be involved in the intestinal effect of morphine while alpha 1- and beta-adrenoceptors do not appear to play any significant role in this aspect of morphine action.     

The possible involvement of alpha-adrenoceptors in the intestinal effect of nalbuphine in mice

In the present study, intestinal motility was measured by the transit of charcoal meal through the small intestine in mice. Nalbuphine, given subcutaneously, caused a dose-dependent inhibition of the intestinal transit in mice. This inhibitory effect of nalbuphine was antagonized by prior s.c. administration of naloxone and MR2266. In addition, the inhibitory effect of nalbuphine was also suppressed by prior administration of yohimbine, an alpha 2-adrenoceptor antagonist, and phentolamine, an antagonist for both alpha 1- and alpha 2-adrenoceptors. Unlike morphine, the intestinal inhibitory effect of nalbuphine was also antagonized by prazosin, a selective alpha 1-adrenoceptor antagonist. However, prior administration of propranolol did not alter this effect of nalbuphine. These adrenoceptor antagonists by themselves, at the doses used, had no effect on the rate of intestinal transit of a charcoal meal in mice. These results suggest that both alpha 1- and alpha 2-adrenoceptors may be involved in the intestinal effect of nalbuphine while beta-adrenoceptors do not appear to play any significant role in this aspect of nalbuphine's action.     

Effect of morphine and naloxone on intestinal transit in mice

Morphine caused a dose-dependent slowing of the rate of intestinal transit in mice. This inhibitory effect of morphine was antagonised by naloxone administration. Pretreatment with a single dose of morphine did not induce any detectable tolerance to the inhibitory effect of a second dose of morphine given 5 h later. However, naloxone was more effective in antagonising this inhibitory effect of morphine-pretreated mice than in saline-pretreated animals. Molecular sieve morphine pellet implantation for 24 h induced detectable tolerance to the inhibitory effect of morphine administered 3 h after removal of the pellet. In addition, the antagonistic effect of naloxone was also augmented when compared with blank pellet-implanted control animals. The present study has shown that the enhanced naloxone potency against the inhibitory effect of morphine on intestinal transit was observable before the development of overt tolerance, and that tolerance to the effect of morphine on the small intestine could be induced by implantation of a molecular sieve morphine pellet for 24 h.     

Antagonist properties of phentolamine at both presynaptic alpha 2-adrenoceptors and presynaptic dopamine receptors using field stimulated right cat atria

Phentolamine, which is considered a specific alpha-adrenoceptor antagonist, was tested for antagonist properties at presynaptic dopamine receptors and presynaptic alpha 2-adrenoceptors present on sympathetic nerve terminals in isolated right cat atria. Field stimulation induced a transient tachycardia which was inhibited by stimulation of presynaptic dopamine receptors using apomorphine or by stimulation of presynaptic alpha 2-adrenoceptors using clonidine. The presynaptic inhibitory effects of apomorphine were competitively antagonized by sulpiride, which is considered a specific dopamine receptor antagonist, and by phentolamine and yohimbine which are considered alpha-adrenoceptor antagonists. The presynaptic inhibitory effects of clonidine were competitively antagonized by phentolamine and yohimbine but not by sulpiride. pA2 values for phentolamine against apomorphine and phentolamine against clonidine suggest that phentolamine may be an antagonist at both presynaptic dopamine receptors and presynaptic alpha 2-adrenoceptors.     

Interaction of opioid drugs with human platelet alpha 2-adrenoceptors.

Morphine, naltrexone and naloxone inhibited the binding of [3H]clonidine to the alpha 2-adrenoceptors in human platelet membranes, provided that Mg2+ was present in the medium. In the presence of 5'-guanylyl imidophosphate (Gpp(NH)p) or in the absence of Mg2+ morphine did not modify the binding of [3H]clonidine. Neither [D-Ala2,N-MePhe4,Gly5-ol]enkephalin (DAGO), nor [D-Pen2,D-Pen5]enkephalin (DPDPE), nor dynorphin-(1-17) affected the [3H]clonidine binding. The presence of 1 mM naloxone did not alter the affinity of either [3H]clonidine or [3H]yohimbine, but reduced the number of binding sites of [3H]clonidine, having no effect on [3H]yohimbine. Naloxone inhibited the binding of adrenaline to high- but not low-affinity sites. It is concluded that morphine and semisynthetic antagonist derivatives interact with alpha 2-adrenoceptors only in the high-affinity state.     

Central and peripheral inhibitory effects of morphine on intestinal transit in mice

In the present study, intestinal motility was measured by the transit of charcoal meal through the small intestine in mice. Morphine, given subcutaneously, caused a dose-dependent slowing of the rate of intestinal transit. This inhibitory effect of morphine was antagonised by prior subcutaneous administration of naloxone hydrochloride or its quaternary analog naloxone methylbromide. However, on a weight basis, naloxone methylbromide was only about half as potent as naloxone hydrochloride. Morphine given centrally, either intracerebroventricularly or intracisternally, caused a dose-related inhibition of intestinal transit, the intracerebroventricular route appearing to be more effective. The effects of centrally administered morphine were antagonised by prior subcutaneous administration of naloxone hydrochloride but not by naloxone methylbromide. The narcotic antagonists administered centrally were effective in suppressing the inhibitory effect of subcutaneously administered morphine. When given intracerebroventricularly naloxone methylbromide was as effective as naloxone hydrochloride, while by intracisternal route, it was about half as potent as naloxone hydrochloride. These results provide evidence that peripherally administered morphine inhibits intestinal transit by both central and peripheral mechanisms. Of the central routes of administration studied, it appears that the sites around the lateral ventricles play a more significant role in the intestinal inhibitory actions of morphine.     

alpha-Adrenoceptor blocking properties of raubasine in pithed rats.

1 Raubasine was compared with yohimbine and corynanthine in pithed rats. Antagonist activity at alpha 1-adrenoceptors was evaluated on the pressor response to electrical stimulation of the spinal sympathetic outflow and to phenylephrine administration, both being reduced by raubasine in the dose range 1 to 4 mg/kg. Corynanthine was quantitatively similar, but yohimbine was not only less potent but also in doses of 0.125 to 0.5 mg/kg enhanced the effects of electrical stimulation. 2 Antagonist activity at alpha 2-adrenoceptors was determined against the inhibitory effects of clonidine on tachycardia induced by electrical stimulation of cardiac sympathetic nerves and against the pressor responses to B-HT-933 injection. Raubasine up to 4 mg/kg, like corynanthine, did not affect the pressor responses to B-HT-933 nor did it reduce the inhibitory effect of clonidine. By contrast yohimbine reduced the response to BHT-933 and antagonized clonidine as well as enhancing the tachycardia caused by electrical stimulation. 3 The results indicate that, in vivo, raubasine, like corynanthine, is a selective antagonist at alpha 1-adrenoceptors and that yohimbine is more potent in blocking alpha 2-than alpha 1-adrenoceptors.     

Studies to determine whether there is tolerance or cross-tolerance to the antisecretory effect of morphine and clonidine in the rat intestine.

Intra-arterial infusions (4 micrograms min-1) of prostaglandin E2 (PGE2) were used to stimulate intestinal fluid secretion in anaesthetized rats. Morphine (0.625-40 mg kg-1) produced a dose-dependent restoration of fluid transport from secretion to the normal rate of absorption. Pretreatment with up to eight doses of morphine (20 mg kg-1) did not induce tolerance, rather it enhanced the antisecretory effect of a subsequent acute dose of morphine. It seems probable that this was caused by the accumulation of morphine in the intestine. Clonidine (4-1000 micrograms kg-1, like morphine, produced a dose-dependent reversal of stimulated fluid secretion. Pretreatment with clonidine (4 x 0.25 mg kg-1) caused a shift of the clonidine antisecretory dose-response curve to the right, demonstrating tolerance. Pretreatment with clonidine also caused cross-tolerance to the antisecretory effect of morphine. The results suggest that there is a close relationship between opioid- and alpha 2-adrenoceptors in controlling inhibition of intestinal fluid secretion.     

Inhibitory action of intracerebroventricular injections of norepinephrine and clonidine on ventricular extrasystoles induced by stimulating hypothalamus in rabbits

The effects of bilateral intracerebroventricular (icv) injections of norepinephrine (NE) and clonidine on hypothalamic stimulation-induced ventricular extrasystoles (VE) were investigated in alpha-chloralose and urethane anaesthetized rabbits. NE and clonidine reduced the number of HVE. The effects of NE were antagonized by pretreatment with phentolamine and yohimbine, but unaffected by pretreatment with prazosin. Pretreatment with yohimbine could also abolish the effects of clonidine. It is concluded that the effects of reduction in VE induced by NE and clonidine may be mediated by activating central alpha 2-adrenoceptors rather than alpha 1-adrenoceptors.     

THE EFFECT OF CHLORPROMAZINE PRETREATMENT ON THE NARCOTIC ANTAGONISTIC POTENCY OF NALOXONE IN MICE

Morphine pretreatment (8.0 mg/kg s.c.) induced no overt tolerance to its antinociceptive effect in mice 4 h later, but enhanced the antagonistic potency of naloxone. Pretreatment with chlorpromazine hydrochloride (0.5–2.0 mg/kg s.c.) potentiated the antinociceptive effect of morphine measured 4.5 h later. The antagonistic effect of naloxone was also enhanced. The observed effect of chlorpromazine on naloxone potency was augmented when naloxone hydrochloride 0.2 mg/kg was administered in the pretreatment regime. The enhanced naloxone potency induced by morphine pretreatment was inhibited by chlorpromazine administered 0.5 h before the morphine pretreatment. These results indicate that pretreatment with either morphine or chlorpromazine increased the antagonistic potency of naloxone. However, it appears that these two drugs act by different mechanisms.     

Antagonism of the emetic action of xylazine by alpha-adrenoceptor blocking agents

The intramuscular injection of xylazine (2 mg/kg) evoked vomiting in 81% of the dogs studied. Adrenoceptor antagonists showing alpha 2-blocking activity, yohimbine, tolazoline and phentolamine, antagonized the xylazine-induced vomiting in a dose-dependent manner. Of these antagonists, yohimbine was the most effective, since the maximal antagonistic effect was seen at 0.5 mg/kg yohimbine, a dose at which the other drugs had less or no effect. The adrenoceptor antagonists showing alpha 1-blocking activity, prazosin and phenoxybenzamine, at the doses studied did not prevent the emesis induced by xylazine. A beta-adrenoceptor antagonists, propranolol, was ineffective in reducing xylazine-induced vomiting. The dopamine receptor antagonists, metoclopramide and domperidone, did not prevent xylazine-induced vomiting nor did yohimbine antagonize apomorphine-induced vomiting. The xylazine-induced vomiting was not prevented by atropine, naloxone or hexamethonium. These results indicate that the xylazine-induced vomiting is mediated by alpha 2-adrenoceptors and does not appear to involve beta-adrenoceptors, cholinoceptors, dopamine or opiate receptors in the emetic pathway.     

Effect of l-ephedrine on cholinergic neurotransmission in the isolated guinea pig ileum

In the isolated guinea pig ileum, the effects of I-ephedrine on the twitch response to field stimulation were investigated in the presence of propranolol. Ephedrine and clonidine inhibited the twitch response but not the contraction to exogenous acetylcholine. The inhibitory effect of clonidine was significantly diminished by yohimbine pretreatment. However, the inhibitory effect of ephedrine was not influenced by yohimbine, sulpiride or 6-hydroxydopamine (6-OHDA) pretreatment. Most of this action of ephedrine appeared to be cholinergic prejunctional in nature, but unrelated to activation of prejunctional alpha 2-adrenoceptors and dopamine sensitive receptors on the cholinergic nerves in this preparation.     

Pharmacological characterization of the postsynaptic alpha-adrenoceptors in human uterine artery

Prazosin and yohimbine were used to differentiate postjunctional alpha-adrenoceptors in the human uterine artery in-vitro. Two postjunctional alpha-adrenoceptor subtypes were distinguished by the affinities of the receptor for yohimbine and prazosin. The pA2 for prazosin was 8.91 against phenylephrine with a slope not significantly different from unity (0.91), and the pA2 for yohimbine was 7.25 against naphazoline and 8.70 against clonidine, with slopes not significantly different from unity (1.11 and 1.18, respectively). Yohimbine was not very active against phenylephrine, while prazosin was very active against the mixed and selective alpha 2-adrenoceptor agonist noradrenaline and clonidine; the intercepts of the Schild plot were 8.80 and 8.82 but with slopes significantly less than unity (0.77 and 0.67, respectively). Prazosin competitively antagonized phenylephrine at the alpha 1-adrenoceptor, whereas yohimbine competitively antagonized naphazoline and clonidine at the alpha 2-adrenoceptor. It is concluded that both alpha 1- and alpha 2-adrenoceptors are present in the human uterine artery.     

Alpha 2-adrenoceptor modulation of nociception in rat spinal cord: location, effects and interactions with morphine

The effects of intrathecal clonidine alone and prior to intrathecal morphine were studied on electrically evoked A beta and C fibre activity in the dorsal horn of the halothane-anaesthetised rat. Clonidine reduced C fibre-evoked activity in a dose-dependent manner, to a maximum 52% inhibition which was reversed by rauwolscine and yohimbine but not naloxone. High doses of clonidine also produced small inhibitions of A fibre-evoked activity. Clonidine potentiated the inhibitory action of intrathecal morphine on electrically evoked C fibre activity but not A fibre activity. In addition, the location of alpha 2-adrenoceptor and opiate binding sites in consecutive sections of rat lumbar cord was investigated using in vitro autoradiography with selective ligands, and it was demonstrated that both opiate and alpha 2-receptor types are present within the same superficial layers of the dorsal horn of the same animal. The results indicate that alpha 2-adrenoceptors and opiate receptors can interact in the modulation of nociceptive transmission in rat spinal cord.     

THE EFFECT OF CHLORPROMAZINE PRETREATMENT ON MORPHINE ANALGESIA AND NALOXONE POTENCY IN ADRENALECTOMIZED MICE

Morphine pretreatment (2.0 mg/kg s.c.) induced no overt tolerance to its antinociceptive effect in mice 3 h later, but enhanced the antagonistic potency of naloxone. 2 Pretreatment with chlorpromazine slightly potentiated the antinociceptive effect of morphine measured 3.5 h later. The antagonistic effect of naloxone was also enhanced. 3 The observed effect of chlorpromazine on naloxone potency was augmented when naloxone was administered in the pretreatment regime. 4 The enhanced naloxone potency induced by morphine pretreatment was inhibited by chlorpromazine administered 0.5 h before the morphine pretreatment. 5 Adrenalectomy sensitized the animals to the antinociceptive effect of morphine. However, the naloxone potency was similar to that of the sham-operated controls. 6 Adrenalectomy greatly attenuated the effect of chlorpromazine pretreatment on naloxone potency. However, the effect of combined pretreatment with either chlorpromazine plus naloxone or morphine remained unaffected. 7 These results indicate that the increased naloxone potency after chlorpromazine pretreatment may be partly mediated through an adrenal-linked process. However, it appears that this process is not essential for the development of increased naloxone potency induced by morphine pretreatment and for the interaction between chlorpromazine, naloxone and morphine.     

Functional postsynaptic alpha 2- but not alpha 1-adrenoceptors in dog saphenous vein exposed to phenoxybenzamine

Phenoxybenzamine greatly attenuated phenylephrine-induced contractions of dog saphenous vein in vitro, but had less effect on contractions induced by clonidine. The phenoxybenzamine-resistant responses to clonidine were not affected by prazosin or by corynanthine but were competitively antagonized by yohimbine (pA2 8.2). It is concluded that exposure of saphenous vein to phenoxybenzamine resulted in blockade of alpha 1-adrenoceptors to the extent that there remained a virtually homogeneous population of postsynaptic alpha 2-adrenoceptors. The effects of agents at postsynaptic alpha 2-adrenoceptors can be studied on this preparation without the complications caused by the presence of functional alpha 1-adrenoceptors.     

Noradrenaline release in rat locus coeruleus is regulated by both opioid and alpha(2) -adrenoceptors.

Extracellular fluid levels of noradrenaline (NA) in the locus coeruleus (LC) during naloxone-precipitated morphine withdrawal with pretreatment of yohimbine (1 mg kg (-1), s.c.) or clonidine (1 mg kg (-1), s.c.) were measured in rats. There was a significant increase in the NA level after the injection of naloxone (2 mg kg (-1), i.p.) in the morphine-dependent rats. Moreover, the NA levels in the LC markedly increased during the 30-60 min following the naloxone (i.p.) challenge in the morphine-dependent rats pretreated with yohimbine. In contrast, the naloxone challenge in morphine-dependent rats pretreated with clonidine notably decreased the levels of NA in the LC. Behavioral signs of withdrawal were observed following the naloxone challenge in the morphine-dependent rats pretreated with yohimbine, with minimal signs in the morphine-infused rats pretreated with clonidine, and none in the saline-infused controls. These results directly suggest that NA increased within the LC after the naloxone challenge in morphine-dependent animals pretreated with yohimbine may be, at least in part, regulated by alpha(2) -adrenoceptors in the LC.     

The effect of morphine pretreatment on hypothermia induced by morphine in mice.

Morphine caused an apparently dose-dependent hypothermia in mice. Co-administration of naloxone antagonised this effect. Pretreatment with a single dose of morphine induced detectable tolerance to the hypothermic effect of a second dose of morphine given 3 h later and naloxone was more effective in antagonising the hypothermic effect of morphine in morphine-pretreated mice than in saline-pretreated animals. The present study has shown that morphine pretreatment can augment the antagonistic effect of naloxone towards the hypothermic action of morphine.     

可乐定局部应用的抗伤害作用

目的 探讨局部应用可乐定的抗伤害反应作用及其耐受性的产生。方法 观察了局部应用可乐定的抗伤害效应、药物处理时间与可乐定抗伤害效应间的关系、提前全身给予纳洛酮或育亨宾对可乐定抗伤害效应的影响及局部应用氯胺酮对可乐定抗伤害耐受发生的影响。结果 在药物局部处理过的小鼠尾部，可乐定产生了浓度相关性的抗伤害效应；局部应用可乐定的抗伤害效应的强度与局部暴露于药液的时间相关；育亨宾完全阻断了可乐定的抗伤害作用，纳洛酮则无明显影响；氯胺酮与可乐定同时局部给药对可乐定抗伤害耐受的产生没有影响。结论 局部应用可乐定可通过激活外周的 α2肾上腺素能受体产生浓度依赖性的抗伤害效应，避免了全身给药的副作用；反复给药能产生抗伤害耐受，局部应用氯胺酮未能阻止这种耐受的发生，说明外周NMDA受体没有参与耐受的发生。